Field of the Invention
The invention relates to a capacitive biometric sensor, in particular, a fingerprint sensor, having a semiconductor sensor chip whose active surface, which can be touched by a person to be identified, is in the form of a sensor array having a large number of sensor electrodes disposed in a rectangular matrix and are covered by a protective layer that extends over the entire sensor array, with the sensor using the electrical pulses from each sensor electrode to produce a local, digitized gray scale value, and, in which case, these gray scale values can be evaluated further in their totality for identification.
Sensors such as these are already commercially available. In particular, the prior art sensor allows fingerprints to be read and evaluated reliably. Wherever the user still has to use personal secret numbers and passwords at the moment, for example, in the case of mobile telephones or PCs, check cards, and credit cards, in the future, the user's own finger will be sufficient. The rectangular sensor array optimizes the identification performance both for production of the reference data record and for each subsequent check. With an area of less than 160 mm2, the sensor can even be used on very small equipment, in particular, even smart cards.
In contrast to the situation with the previously existing optical scanners, the capacitive sensor does not operate by “taking photographs” but substantially based upon 65,000 sensor electrodes, which are disposed alongside one another on the sensor array, and each represent one pixel, with the exact distance between the skin surface and the sensor respectively being measured locally when a finger is placed on the sensor array, thus, mapping the skin structure. The mapping is accomplished by utilizing the capacitance difference between the lines and troughs in the fingerprint, as results when a finger is placed on it, whose different points in each case form the opposing electrode for the sensor electrode that is located underneath such a point.
After less than 100 milliseconds, the sensor produces a digitized gray scale image of the fingerprint with a lateral resolution of 20 individual points per millimeter (513 dots per inch). Image processing software, then, evaluates the gray scale image based upon the characteristic features of the fingerprint, the so-called minutiae, and stores their appearance, position, and alignment. During the subsequent check, an evaluation algorithm compares the sensor image with the previously stored reference data. In a typical application, the enable signal for the sensor chip for activation of an item of equipment is produced only if the two data records match.
Correct use of the sensor is governed not only by the already mentioned horizontal fineness of the resolution in the plane of the sensor array, but, likewise, by depth or contrast resolution of each individual pixel that can be distinguished in this way. The 8-bit data resolution of each pixel results in a spectrum of 256 gray scale levels, with the gray scale value 0 in such a context representing “black” and the gray scale value 255 representing “white.” An actual gray scale image of the fingerprint never covers this entire bandwidth, however. The maximum available local contrast is, in practice, in fact governed by the maximum difference between the first gray scale value, which corresponds to a finger line (skin moisture) and the second gray scale value, which corresponds to a finger trough (air). Normally, however, the frequency distribution of a number of individual contrast resolutions is considered in the form of a histogram. The distance between the two prominence maxima (that is to say, the skin moisture or water, on one hand, and air, on the other hand) is referred to as the dynamic resolution of the sensor.
At the moment, the dynamic resolution is determined either directly by placing a finger on the sensor, or by mechanical aids, which, likewise, involve contact. One suitable way to measure the so-called “water line” is, in principle, to fill the sensor array with water, although, in practice, a test stamp is normally used. The “air line” must be recorded in a second measurement, in which the sensor array is not covered.
The check of the dynamic resolution of each individual sensor chip should be carried out, in particular, at the end of the production process, especially if unserviceable sensor chips are not intended to be identified and possibly filtered out by complex electrical tests, but simply by checking or determining the central functional element of “dynamic resolution”. However, previous capacitive sensors make it necessary to use the already described complex way of determining the dynamic resolution.